A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to make use of an encoder measurement system in order to measure a position of a stage (such as a substrate stage or patterning device stage) in the lithographic apparatus. Thereto, a (e.g., two dimensional) encoder grid is applied to a first part of the lithographic apparatus, while encoder sensors heads are connected to a second part of the lithographic apparatus. In an embodiment the encoder grid is connected to a reference structure of the lithographic apparatus, while encoder sensor heads are connected to the stage so as to follow its position. In another embodiment the encoder grid is connected to the stage and the encoder sensor heads are connected to the reference structure. In order to calibrate the encoder measurement system, a plurality of calibrations is performed, which may result in a long calibration time and may result in loss of accuracy by stitching together the different calibration results for the various frequency bands.
In applications of a lithographic apparatus marks are used for different detection methods such as alignment and overlay measurements. More detailed information with respect to a substrate and performance of the lithographic apparatus is desirable and detecting a relevant property is becoming more and more time consuming.